Move IceCandidate::ToString implementation to api

This commit moves the `IceCandidate::ToString()` implementation
from `pc/jsep_ice_candidate.cc` to `api/jsep_ice_candidate.cc`.

This is a follow-up to a previous refactor that removed the
dependency on `webrtc_sdp`. With that dependency resolved,
`ToString()` can now live in the same build target as the
`IceCandidate` class definition and its other member functions.

The implementation of `ToString()` is also updated to call
`candidate_.ToCandidateAttribute(true)` directly, removing its
reliance on `SdpSerializeCandidate`.

As part of this cleanup, related serialization functions in
`pc/webrtc_sdp.cc` (`SdpSerializeCandidate` and `BuildCandidate`)
are simplified to remove duplicated logic and use the canonical
`ToCandidateAttribute` method.

Bug: webrtc:12330
Change-Id: I9babf11ec72f581e8a1167aa6d1e6b32d42c6cb9
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/411041
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#45779}